Chromium is a base for a large number of materials with very high antiferromagnetic (AF) transition temperatures. This fact has stimulated their study as high Tc superconductivity deve lops from antiferromagnetic parents. Most interesting are the two-dimensional oxides of the Æn+1CrnO3n+1 Ruddlesden-Popper series which are poorly studied due to their extreme difficult synthesis conditions. Using the large volume presses available in the Néel Institute, I have synthesized the n=1 and 2 of the Sr series and studied their previously unknown physical properties in details. In Sr3Cr2O7 I have found the appearance of AF order below TN=210K. The magnetization measurements were confirmed by neutron scattering and the AF structure was calculated from them. Specific heat measurements show a very large anomaly at TN, with an entropy much larger than expected from magnetic ordering alone. Electronic band structure calculations and theoretical models built on our data, suggest a total magnetic and orbital ordering coupled to a specific lattice distortion as observed in our neutron diffraction measurements. Sr2CrO4 develops AF at a higher temperature TN=305K, both from magnetization and neutron diffraction measurements. The latter also show new magnetic peaks appearing below 150K, typical of a magnetic moment rearrangement. It is coupled to an unusual "Anti-Jahn-Teller" effect, i.e. the deformed CrO6 octahedron becoming un-deformed at low temperatures. The interpretation of this unexpected result is under study. Recently, I have also synthesized the n=1 and 2 member of the Ca-based family that present roughly similar properties. Finally, I will briefly describe the pressure phase diagram of another Cr-based compound, CrSb2, that I have determined, following the report of superconductivity at Tc = 2 K under 0.8 GPa of pressure in CrAs.